Optical Curiosities of Literature. 22!) 



our attention very much more than has yet been done to illumina- 

 tion atid its interpretation. Things seen with high powers and 

 large apertures ape seen under conditions essentially difl'ering from 

 those ot ordinary vision ; so that in these realms it ceases to be 

 true that seeing is believing. And when the best has been done 

 for our glasses, we must still, I fear, supplement the discernment 

 of our eyes very largely by the discernment of our reason. And 

 this of course involves chiefly the question of illumination and its 

 illusions. At present the methods we possess are three — opaque, 

 direct-transmitted, and dark-field. The lens in question has now 

 added another which in nature and principle differs from all these, 

 and supplies us with what I may call a new analysis of the objects 

 as seen. But to use it so as to make it an analysis it is even more 

 necessary than in the other methods that we understand the prin- 

 ciple on which it works. I therefore append a diagram, which may 



help to effect this by showing the action reduced to its most simple 

 form. The lens and slide are here represented as forming a single 

 piece, which optically they do (from the intervening oil). The 

 object rests on the slide, touching it here only at a single point. 

 The cover is omitted intentionally, having nothing at all to do with 

 the work. It may no doubt be placed over the object, it is of no 

 consequence at all whether or not ; provided always that if placed 

 no fluid is added between, and the object must still be, as here, on 

 the slide. For simplicity therefore I omit it entirely. All the light 

 which enters the slide is lost by total reflexion, except the ray which 

 reaches the single point of contact. This ray is transmitted by enter- 

 ing within the object itself, and is there dissipated, more or less, 

 according to its structure. This method therefore is distinct in nature 

 from those before in use. Mr. Wenham has given no name to it. 

 I have been accustomed to call it Internal Illumination. It is seen 

 at once that it gives an analysis of the contact of the object with 

 the slide. It might be thought the illumination would be symme- 



